1. Field of the Invention
This invention relates to a method for the purification of a gas and a catalyst for use in the purification. More particularly, it relates to a method for catalytically decomposing ozone contained in a gas or catalytically decomposing a component of offensive odor in a gas through oxidation with ozone and to a catalyst used for the decomposition.
2. Description of the Prior Art
Ozone possesses a strong oxidizing ability and, on decomposition, converts itself into harmless oxygen. Thus, it has found extensive utility in various applications aimed at deodorizing, sterilizing, and bleaching various substances and even lowering the COD level of plant effluents. Unfortunately, since part of the ozone so used in varying treatments is released in its unaltered form into the air, the stray ozone has the possibility of giving rise to secondary environmental pollution such as, for example, generation of a photochemical smog. In the case of an airplane in flight through the stratosphere, since the air containing ozone is drawn into the interior of the airplane, the passengers and the crew aboard the airplane may possibly be exposed to the adverse effect of the ozone in the air.
Further, the issue of ozone from machines incorporating therein various high voltage generating devices such as, for example, xerographic copying machines has recently been posing a problem. Since these machines are installed mainly in rooms, the ozone so generated pollutes the interiors of the rooms, though the amount of the ozone so generated is very small.
The odor of ozone can be perceived even when the ozone is present in a concentration of not more than 1 ppm. When ozone is present in the air in a concentration exceeding 2 ppm, it stimulates the respiratory system of a person inhaling the air and does harm to the person's body. The air which contains the ozone issuing from a varying ozone sources must be detoxified by removal of the ozone.
As conventional techniques used for disposal of spent ozone, the method using activated carbon, the method using a liquid detergent, and the method resorting to thermal decomposition have been available When the activated carbon method is utilized for the treatment of air containing ozone in a low concentration, the activated carbon must be replenished as it is consumed in consequence of progress of the decomposition of ozone. When this method is used for the treatment of air containing ozone in a high concentration, the activated carbon must be handled attentively because it has the possibility of being ignited and burnt by the heat of reaction.
The liquid detergent method resides in purifying air containing spent ozone with the aqueous solution of a reducing substance. Thus, the treatment costs much and gives rise to a problem of waste water disposal.
The thermal decomposition method has the disadvantage that it requires application of heat to a temperature of not less than 300.degree. C. for the purpose of enhancing the efficiency of decomposition, necessitates an expensive application of heat for the disposal of a large volume of waste gas, and engenders a high cost of treatment.
In recent years, as one approach to the disposal of spent ozone, the method which resorts to catalytic decomposition has been the subject of research. It is held that this method constitutes an advantageous way of removing ozone by decomposition at a low cost because it has no possibility of inducing ignition or explosion and obviates the necessity for waste water disposal.
Regarding the decomposition of ozone, a catalyst using the oxides of nickel, manganese, cobalt, etc. is disclosed as exhibiting an outstanding ability to effect efficient decomposition of ozone in the specification of Japanese Patent Laid-Open SHO 60(1985)-97,049. For practical reasons, however, the necessity for developing a catalyst exhibiting high activity in the range of much lower temperatures has been finding growing recognition.
In recent years, the pollution of air with offensive odor has arisen as a serious social problem and thus the need for development and adoption of new techniques for combatting such odors.
Heretofore, deodorization of foul air has been carried out by the method of washing with a detergent, the method of adsorption, the method of direct combustion, the method of catalytic combustion, the method of oxidation with ozone, etc. These methods invariably have both merits and demerits of their own and leave much to be desired. The detergent method yields a large volume of waste water and incurs a large cost for the disposal of waste water. The adsorption method which mostly uses activated carbon as an adsorbent has the disadvantage that devices such as for replacement of activated carbon are difficult to maintain because the adsorbent is liable to catch fire and is quickly deprived of the deodorizing effect. The direct combustion method requires to use fuel and, therefore, entails a high running cost, demands a careful consideration on safety of operation, and suffers a great expense for a voluminous plant. In the case of the catalytic combustion method, though the plant can be maintained relatively easily, the temperature of the catalyst bed must be maintained in the range of 300.degree. to 450.degree. C. When the gas to be treated has a low temperature or the flammable substance is present in a low concentration, therefore, this method suffers from the disadvantage that the running cost is inevitably high. The ozone oxidation method effects elimination of a component of offensive odor by making use of the powerful oxidizing action of ozone. Since this method effects the treatment at a low temperature in the neighborhood of normal room temperature, it enjoys a low operating cost as compared with the various methods mentioned above Since the reaction of ozone with the component of offensive odor in the gaseous phase proceeds slowly, this method necessitates adoption of a long and voluminous reactor The method further suffers from the disadvantage that the unaltered portion of ozone is released into the atmosphere possibly to induce secondary air pollution.
Two new methods have been proposed as means of overcoming the aforementioned drawbacks of the various conventional methods The first method effects deodorization of a foul gas by the use of an apparatus provided with an ozone generator and an ozone decomposition filter (Japanese Patent Laid-Open SHO 61(1986)-29,358.)
This method has no possibility of causing secondary air pollution because the unaltered portion of ozone is forcibly decomposed before it is released into the atmosphere. Since both the unaltered portion of ozone and the component of offensive odor are decomposed in the gas phase, the reactor to be used is required to possess a large capacity for the reason mentioned above. If the reactor has a small capacity, there ensues the disadvantage that no sufficient deodorizing effect is obtained because the foul gas under treatment passes the ozone decomposition filter before it is thoroughly deodorized.
The second method resides in using a catalyst for the purpose of accelerating the reaction of ozone with the component of offensive odor thereby ensuring thorough oxidation of the component of offensive odor and, at the same time, effecting catalytic decomposition of the unaltered portion of ozone.
As catalysts available for this method, a catalyst having a metal oxide deposited on a carrier made of a carbonaceous material (Japanese Patent Laid-Open SHO 54(1979)-119,371) and a catalyst having a metal oxide deposited on a carrier of activated alumina (Japanese Patent Laid-Open SHO 53(1978)-30,978) have been disclosed.
In the case of the former catalyst, the carrier is consumed by the combustion of carbon with ozone and the carrier possesses such a large adsorbing capacity as to adsorb the product of oxidation and, as an inevitable result, the catalyst suffers from rapid deterioration and fails to offer an adequate service life.
The latter catalyst cannot be expected to offer an adequate service life because sulfur compounds (such as methyl mercaptan and hydrogen sulfide) which are representative ingredients of the component of offensive odor are oxidized into SO.sub.3 and this product of oxidation is deposited cumulatively on the active alumina of the carrier.
As described in detail above, with the conventional methods, it is difficult to attain deodorization to a thorough extent and effect substantial elimination of the leakage of the unaltered ozone into the atmosphere.
An object of this invention, therefore, is to provide a novel method for the purification of a gas and a catalyst to be used therefor.
Another object of this invention is to provide for catalytic decomposition of ozone contained in a gas into oxygen, an inexpensive ozone decomposition catalyst having excellent low-temperature activity and ability to remove the offensive odor component of a gas by catalytic oxidative decomposition of the said component with ozone, a method for inexpensive deodorization capable of producing a stable and efficient deodorizing effect over a long period and substantially eliminating release of the unaltered portion of ozone into the atmosphere.